A conventional power converter circuit 10 has, as shown in FIG. 21, a coil L having one end connected to a DC power source 2, a high side switch SW1 and a low side switch SW2, which are connected to another terminal of the coil L, and a capacitor C that is connected to ends of the respective switches SW1 and SW2 at an opposite side of the coil L.
In the power converter circuit 10 of the above type, the high side switch SW1 and the low side switch SW2 which constitute a chopper circuit alternately turn on and off to change over a path of a current that flows in the coil L, thereby controlling an output voltage Vout produced from the capacitor C.
In the power converter circuit 10 of the above type, when the high side switch SW1 and the low side switch SW2 turn on at the same time in conducting output control, a through-current may flow in the chopper circuit due to an electric power that is stored in the capacitor C, resulting in a damage of the respective switches SW1 and SW2.
For this reason, in the power conversion device having the power converter circuit 10 of the above type, a dead time for turning off the respective switches SW1 and SW2 at the same time is given a drive signal of the respective switches SW1 and SW2 so that the switch SW2 (or SW1) which is in an off state turns on after the switch SW1 (or SW2) which is an on state turns off when the on/off states of the respective switches SW1 and SW2 changes over.
That is, the power conversion device shown in FIG. 21 represents a DC/DC converter that S up an input voltage Vin from the DC power source 2 through the power converter circuit 10 to generate a drive voltage of an electric load 4. The power conversion device includes an A/D converter 12 that converts the output voltage Vout that is a drive voltage of the load 4 into a digital value, a deviation calculation unit 14 that calculates a deviation between the output voltage Vout that has been converted into a digital value by the A/D converter 12 and a command value (that is, a target voltage) representative of a target value of the output voltage Vout, a switching ratio calculation unit 16 that calculates the switching ratio of the power converter circuit 10 necessary to make the deviation zero based on the deviation that has been calculated by the deviation calculation unit 14 and a control gain (a control gain such as proportion/integration/differential), and a PWM signal generation unit 19 that generates a PWM signal for turning on and off the high side switch SW1 and the low side switch SW2 based on the switching ratio that has been calculated by the switch ratio calculation unit 16, respectively.
Then, the PWM signal generation unit 19 sets both of the PWM signals to the respective switches SW1 and SW2 to low level for a predetermined dead time Td. This prevents the respective switches from turning on by a response delay at the time of turning off the respective switches SW1 and SW2, simultaneously, when the PWM signal generation unit 19 generates the PWM signals for driving the respective switches SW1 and SW2, respectively (FIG. 22).
For this reason, according to the power conversion device shown in FIG. 21, the through-current can be prevented from flowing in the chopper circuit by turning on the respective switches SW1 and SW2, simultaneously, at the time of changing over the on/off states of the high side switch SW1 and the low side switch SW2.
a current flows in the coil L through parasitic diodes of transistors (MOSFETs in the figure) which constitute the respective switches SW1 and SW2 for the dead time Td during which both of the PWM signals to the respective switches SW1 and SW2 are low in signal level. The current direction is different according to the fluctuation region of the coil current under the control.
That is, as exemplified in FIG. 22, an increase/decrease direction of the coil current Ic that flows during the dead time period Td (shaded region in FIG. 22) after the high side switch turns off is different between when the coil current Ic is a positive current value within the control cycle period (Imin≧0) and when the coil current Ic varies from positive to negative or from negative to positive (Imin<0<Imax).
In FIG. 22, Imin represents the minimum value of the coil current, and Imax represents the maximum value of the coil current. When the coil current Ic is positive which is larger than 0 A, the current flows in the coil L from the DC power source 2 side to the connection point side of the switches SW1 and SW2. When the coil current Ic is negative which is smaller than 0 A, the current flows in the coil L to the DC power source 2 side from the connection point side of the switches SW1 and SW2.
When the ratio of the dead time period Td to one cycle of the PWM signal (that is, control cycle period Tcp) is small even if the increase/decrease direction of the coil current Ic during the dead time period Td is different as described above, there arises substantially no problem. However, when the ratio of the dead time period to the control cycle period is larger with the higher switching frequency, the following problem occurs. That is, as shown in FIG. 23, the increase/decrease direction of the coil current Ic that flows during the dead time Td changes to deviate the coil current Ic from an ideal current Ii. As a result, the output voltage is temporarily largely deviated from the target voltage.
That is, FIG. 23 shows a change in the coil current Ic when the load 4 varies from −200 W to +200 W in the case where the PWM signal generation unit 19 subtracts the dead time Td from an on-time TH of the high side switch SW1 and an on-time TL of the low side switch SW2, respectively, to generate the PWM signals PWM(L) and PWM(H) for driving the high side switch SW1 and the low side switch SW2, respectively. The on-time TL of the low side switch SW2 and the on-time TH of the high side switch SW1 correspond to the switch ratio (ratio command value) Rsw that has been calculated by the switching ratio calculation unit 16.
As is apparent from FIG. 23, the coil current Ic changes in correspondence with the PWM signal that is generated by the PWM signal generation unit 19 when the coil current Ic changes around 0 A within the control cycle period (Imin<0<Imax). When the coil current Ic is equal to or lower than 0 A over the entire region of the control cycle period (Imax≦0), or when the coil current Ic is equal to or higher than 0 A over the entire region of the control cycle period (Imin≧0), the ratio of the increase/decrease in the coil current Ic does not correspond to the ratio command value, the coil current Ic cannot change along the ideal current Ii, and the output voltage Vout is deviated from the target voltage.
On the other hand, in order to prevent the above problem, for example, JP 2004-120844A) proposes that, as indicated by dotted lines in FIG. 21, the output voltage Vout and the output current to the load 4 from the power converter circuit 10 is detected to obtain the output power that is actually supplied to the load 4 from the power converter circuit 10. In this proposal, the output power is then compared with plural threshold values (comparison unit 82) to determine a path of the coil current within the power converter circuit (a variation region: Imax≦0, Imin<0<Imax, Imin≧0). The correction value of the switching ratio (ratio command value) is obtained based on the determination result (correction value calculation unit 84), and the switching ratio (ratio command value) is corrected by using the correction value (correction unit 18).
However, the above proposed device estimates the variation region (Imax≦0, Imin<0<Imax, Imin≧0) of the coil current within the power converter circuit 10 based on the output power from the power converter circuit 10 to the load 4 to correct the switching ratio. Therefore, the proposed device is incapable of precisely determining the variation region (Imax≦0, Imin<0<Imax, Imin≧0) of the coil current within the power converter circuit 10 based on the output power during a transition period, where the power consumption on the load side rapidly changes from positive to negative, or vice versa. As a result, as shown in FIG. 24, the coil current Ic changes earlier than the ideal current Ii.
That is, the coil current Ic changes as shown in FIG. 24, when the load power changes from −200 W to +200 W in a short time in the case where the comparison unit 82, the correction value calculation unit 84, and the correction unit 18 in FIG. 21 are configured as follows.
(1) the on-times of the respective switches SW1 and SW2 are set to reference times “TL−Td” and “TH−Td” obtained by subtracting the dead time Td from the on-times TL and TH corresponding to the switching ratio (ratio command value) when the power consumption (output power) of the load 4 is in a range of from −150 W to +150 W,
(2) the on-time of the switch SW1 is set to be shorter than the reference time “TL−Td” by the dead time Td (TL−2Td), and the on-time of the switch SW2 is set to be longer than the reference time “TH−Td” by the dead time Td (TH) when the output voltage is lower than −150 W, and
(3) the on-time of the switch SW1 is set to be longer than the reference time “TL−Td” by the dead time Td (TL), and the on-time of the switch SW2 is set to be shorter than the reference time “TH−Td” by the dead time Td (TH−2Td) when the output voltage is larger than +150 W.
An increase/decrease direction of the coil current that flows in the coil L during the dead time period Td is changed after electric charges have been stored in the capacitor C are gradually discharged, and the output voltage Vout becomes lower than the command value with an increase in the power consumption of the load 4 (a current that flows in the load).
For this reason, when the fluctuation region (Imax≦0, Imin<0<Imax, Imin≧0) of the coil current is estimated from the power consumption (output power) of the load 4 to change the dead time Td that is given to the PWM signals of the respective switches SW1 and SW2, as shown in FIG. 24, the increase/decrease ratio (TL/(TL+TH)) of the coil current cannot correspond to the switching ratio (ratio command value).